Molten salts are salts that are in a molten state, and known examples of the molten salts cover a wide temperature range, i.e., vary from (i) slag melted at a high temperature to (ii) a room-temperature molten salt that liquefies at room temperature. Use of a molten salt as an electrolyte liquid makes it possible to cause an electrochemical reaction that is hard to cause with use of an aqueous electrolyte liquid, and various studies are currently under way in respective fields. Since molten salts can be given functionalities that vary depending on combinations of cations and anions, a wide variety of salts are currently under development for the purpose of various applications.
Known examples of anions that yield a salt having a relatively low melting point among those salts mentioned above include bis(trifluoromethylsulfonyl)amide anion (commonly called “imide anion”, TFSI—, N(SO2CF3)2—, after-mentioned chemical formula (1)). The history of the TFSI anion evolves from the 1990 report of Armand et al. on lithium bis(trifluoromethylsulfonyl)amide (LiTFSI). Since an electrolyte obtained by constructing a composite of LiTFSI and a polymer such as polyethyleneoxide (PEO) exhibits excellent properties as a lithium-ion secondary battery electrolyte, a large number of studies of the electrolyte as an electrolyte supporting salt have been under way up to the present date. Further, it is in 1996 that a room-temperature molten salt obtained by combining an imidazolium cation and a TFSI anion was reported.
In general, since a larger ion will have a larger Stokes radius, the mobility of an ion in a liquid tends to be low, so that conductivity becomes smaller. However, although a TFSI salt has a TFSI anion that is a relatively large anion, it exhibits a high conductivity. The reason for this is as follows: The TFSI anion has two highly electrically-negative CF3SO2 groups and contains charges nonlocalized on nitrogen atoms, and therefore becomes weakly associated with a cation, thereby causing an increase in ion concentration effective in charge transport. The same is equally true in cases where the TFSI anion is used as a counter anion of a room-temperature molten salt. For example, 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide (EMImTFSI) exhibits a conductivity of 8.8 mS cm−1 at room temperature. Furthermore, the TFSI salt also exhibits high electrochemical stability, and therefore is applied advantageously to the field of electrochemistry and expected to be applied as an electrolyte liquid for batteries and capacitors.
Meanwhile, salts MTFSI (M=Li, Na, K, Cs) each containing an alkali metal as a cation while similarly having a TFSI anion are solid at room temperature, but have a melting point in an intermediate temperature range (of not more than 300° C.). If the properties supposed to be exhibited by a TFSI anion in a room-temperature molten salt are preserved even in this temperature range, these MTFSI salts can be applied as an electrolyte useful in an intermediate temperature range, and can be expected to be applied to various fields of electrochemistry. However, although there have been reports on their crystal structures (LiTFSI, KTFSI, CsTFSI, LiTFSI.H2O, NaTFSI.H2O.MeOH) (see Non-patent Document 1), these salts have never been studied in detail as molten salts.
As described above, the application of LiTFSI as a lithium-ion secondary battery electrolyte supporting salt has been widely studied. However, the properties of simple LiTFSI as a molten salt have been hardly studied. Further, as with a composite of LiTFSI and a polymer, a composite of each of NaTFSI and KTFSI and a polymer has been examined; however, there are only a very small number of reports on NaTFSI and KTFSI (see Non-patent Documents 2 and 3). Further, there are no detailed reports on properties such as a melting point. As for CsTFSI, there are neither relevant studies nor reports on its properties, except for the aforementioned report on its crystal structure. Further, there are no reports on RbTFSI.
[Non-patent Document 1]    L. Xue, et al., Solid State Sciences 4 (2002) p 1535
[Non-patent Document 2]    C. Roux, H.-Y. Sanchez, Electrochim. Acta 40 (1995) p 953
[Non-patent Document 3]    A. Ferry, M. M. Doeff, L. C. Jonghe, Jelectro. Chem. 145 (1998) p 1586